1. Field of the Invention
This invention relates to systems for storing large quantities of data in digital audio tape (DAT) or other format, and more particularly to such systems utilizing an automatic cassette loader mechanism.
2. Description of Related Art
Magnetic tape storage devices are widely used for the storage of large amounts of digital data, because they provide an economical and reliable means for temporary and permanent storage. Because magnetic tape systems inherently rely on serial recording, access times are substantially longer than other modern storage devices, but at the same time the danger of catastrophic failure is virtually absent. Thus it has become common practice to utilize tape systems as data backup for hard disk files, typically by reading out the entire contents of a random access memory system at the end of the day or other operating period, and retaining this data in storage until the next backup date or time. Where the volume of data is limited, one tape system and tape reel or cartridge may suffice, but where the data base is much larger, many reels or cartridges may be needed. To utilize a backup system efficiently, it is preferred to record the backup data at a high data transfer rate during what would normally be down time for the system, e.g. the time between the close of business one evening and the start of business the next morning.
Tape drive systems have evolved over the recent past with technical improvements that have resulted in substantial increases in capacity accompanied by significant decreases in size. Large self-contained tape transports using parallel track recording techniques and relatively wide tape have been used, but these are incompatible in size, cost and power requirements with the compact and highly efficient central processor units and disk drives that are now employed. Threading of tape in reel-to-reel devices has always been a cumbersome task, so that efficient tape cassette and cartridge systems have been generated, using longitudinal scan recording techniques. By improvements in recording techniques and efficient cartridge configurations, these tape drives have been made in geometries and with sizes suitable for utilization with a standard peripheral equipment slot in a console, such as the full height slot for receiving a 51/4" floppy disk drive.
More recently, helical scan recording techniques originally devised for video recording have been adapted in compact systems to provide high density, high fidelity, digital audio tape recordings. The DAT format has in turn been adapted, under the so-called Sony/Hewlett-Packard standard, to digital data processing applications. The cassette (also called a cartridge) used for these applications is very small, the standard 4 mm digital audio cassette being 0.39" (10 mm) in height by 2.1" (54 mm) in depth, with a nominal tape width of 0.15" (3.81 mm). The recording technique used is group code encoding with error correcting codes, to the "DDS" specification X3B5/88-185A. "DDS" is a trademark of Hewlett-Packard/Sony. Using helical scan technology and 61K bpi linear density, each cassette has a data capacity of 2.0 Gbytes so that at a sustained transfer rate of 183 Kbytes/sec. (burst transfer rate of 4.0 Mbytes/sec.) there is a capacity for receiving 3.0 hours of data, equivalent to the contents of three large 650 Mbyte disk drives. Large commercial organizations, however, may have many such drives, in a data base system. Consequently large main frame and parallel processors need to use many backup cassettes, even in the DAT format. This is not only more costly but less efficient. It is preferable to utilize a single drive to prepare a number of tape records in sequence, rather than to employ a number of drives in parallel. Thus stacker-type loaders have been considered that fit on the front of the console in which the tape drive is mounted, as add-on units, to provide a handler for the cassettes. This approach is unsatisfactory for aesthetic, safety and technical reasons, and is often unusable simply because of the location of the tape drive unit, since there is usually inadequate clearance available relative to other parts of the computer console.